UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 
AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


FURTHER  EXPERIMENTS  IN 
PLUM  POLLINATION 


BY 

A.  H.  HENDRICKSON 


A  tent  of  white  mosquito  bar  was  built  over  a  pair  of  adjoining  Formosa 
and  Wiekson  plum  trees.  A  hive  of  bees  was  enclosed  in  the  tent  for  six  days 
during  the  blossoming  season  to  test  the  value  of  the  honey  bee  as  a  pollen 
distributor. 


BULLETIN  No.  352 

December,  1922 


UNIVERSITY  OF  CALIFORNIA  PRESS 

BERKELEY,  CALIFORNIA 

1922 


David  P.  Barrows,  President  of  the  University. 

EXPERIMENT  STATION  STAFF 

HEADS  OF  DIVISIONS 
Thomas  Forsyth  Hunt,  Dean. 
Edward  J.  Wickson,  Horticulture  (Emeritus). 

,  Director  of  Resident  Instruction. 

C.  M.  Haring,  Veterinary  Science,  Director  of  Agricultural  Experiment  Station. 

B.  H.  Crocheron,  Director  of  Agricultural  Extension. 

C.  B.  Hutchison,  Plant  Breeding,  Director  of  the  Branch  of  the  College  of 

Agriculture  at  Davis. 
H.  J.  Webber,  Sub-tropical  Horticulture,  Director  of  Citrus  Experiment  Station. 
William  A.  Setchell,  Botany. 
Myer  E.  Jaffa,  Nutrition. 
Ralph  E.  Smith,  Plant  Pathology. 
John  W.  Gilmore,  Agronomy. 
Charles  F.  Shaw,  Soil  Technology. 
John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 
Frederic  T.  Bioletti,  Viticulture  and  Fruit  Products. 
Warren  T.  Clarke,  Agricultural  Extension. 
Ernest  B.  Babcock,  Genetics. 
Gordon  H.  True,  Animal  Husbandry. 
Walter  Mulford,  Forestry. 
James  T.  Barrett,  Plant  Pathology. 
Fritz  W.  Woll,  Animal  Nutrition. 
W.  P.  Kelley,  Agricultural  Chemistry. 
H.  J.  Quayle,  Entomology 
Elwood  Mead,  Rural  Institutions. 
H.  S.  Reed,  Plant  Physiology. 
L.  D.  Batchelor,  Orchard  Management. 
W.  L.  Howard,  Pomology. 
*Frank  Adams,  Irrigation  Investigations. 

C.  L.  Roadhouse,  Dairy  Industry. 
R.  L.  Adams,  Farm  Management. 

W.  B.  Herms,  Entomology  and  Parasitology. 
John  E.  Dougherty,  Poultry  Husbandry. 

D.  R.  Hoagland,  Plant  Nutrition. 
G.  H.  Hart,  Veterinary  Science. 

L.  J.  Fletcher,  Agricultural  Engineering. 
Edwin  C.  Voorhies,  Assistant  to  the  Dean. 

DIVISION  OF  POMOLOGY 

W.  L.  Howard  G.  L.  Philp 

W.  P.  Tufts  L.  C.  Barnard 

E.  L.  Overholser  W.  P.  Duruz 
A.  H.  Hendrickson  C.  L.  Austin 

F.  W.  Allen  M.  J.  Heppner 
J.  P.  Bennett  L.  H.  Day 


*  In  cooperation  with  Division  of  Agricultural  Engineering,  Bureau  of  Public  Roads,  U.  S. 
Department  of  Agriculture. 


FURTHER  EXPERIMENTS  IN  PLUM  POLLINATION 

By  A.  H.  HENDRICKSON 


A  previous  report*  of  the  experiments  on  the  pollination  of  certain 
shipping  varieties  of  plums  contained  data  on  a  few  of  the  older  kinds 
of  shipping  plums  and  on  the  important  varieties  of  prunes.  The 
most  successful  combinations  tested  at  that  time  were  given,  and  the 
importance  of  the  common  honey  bee  as  a  distributor  of  pollen  in 
prune  orchards  was  demonstrated.  The  present  report  gives  the 
results  obtained  from  three  years7  work  at  Vacaville,  Solano  County, 
and  near  Newcastle,  Placer  County,  on  other  varieties,  many  of  which 
have  recently  become  prominent.  It  also  further  emphasizes  the  value 
of  honey  bees  in  orchards  of  shipping  plums. 

OUTLINE    OF    EXPERIMENTS 

Essential  details  of  the  technique  of  the  methods  used  were 
described  in  the  previous  report.*  The  work  was  undertaken  chiefly 
to  obtain  data  on  the  effectiveness  of  certain  combinations  of  varieties 
of  plums  in  producing  satisfactory  crops.  Hence,  if  the  fruit 
remained  on  the  tree  to  maturity,  the  cross  was  considered  successful 
whether  a  viable  seed  was  produced  or  not.  Varieties  able  to  set  and 
mature  fruit  when  pollinated  with  their  own  pollen  are  considered 
in  this  discussion  as  self -fertile.  If  unable  to  set  fruit  with  their  own 
pollen,  they  are  considered  as  self -sterile.  Two  varieties  which  cannot 
be  crossed  one  upon  the  other  so  as  to  produce  fruit  are  considered 
as  inter-sterile. 

The  work  in  1920  was  carried  on  in  the  orchards  of  Mr.  H.  B. 
Naylor  and  Mr.  Millard  Sharpe  of  Vacaville,  California.  The  former 
orchard  is  located  on  a  hillside  several  hundred  feet  above  the  valley 
floor.  The  latter  is  in  the  valley  proper,  in  the  center  of  a  large  fruit- 
growing area.  In  both  cases  the  varieties  of  plums  used  were,  for  the 
most  part,  topworked  on  old  apricot  trees.  In  1921  and  in  1922  the 
work  was  conducted  on  what  was  known  as  the  Cook  ranch,  owned 
by  Mr.  J.  F.  Dudley,  about  two  miles  south  of  Newcastle  in  Placer 
County.  In  the  Cook  orchard,  situated  at  an  elevation  of  about  600 
feet  in  the  Sierra  foothills,  the  soil  is  a  decomposed  granite,  typical 


*  Hendrickson,  A.  H.,  Plum  Pollination.      Calif.  Agr.  Exp.  Sta.  Bull.   310, 
July,  1919. 


248  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

of  a  large  area  in  that  district.  The  trees  were  of  various  ages  and 
were  practically  all  on  peach  root.  Many  had  previously  been  top- 
worked  at  least  once  when  the  first  graft  proved  unsatisfactory.  The 
Vacaville  orchards  were  not  irrigated.  In  Placer  County  the  trees 
were  irrigated  in  shallow  furrows  about  every  week  or  ten  days  during 
the  growing  season.  In  both  districts,  during  the  years  the  experi- 
ments were  carried  on,  the  trees  for  the  most  part  blossomed  heavily, 
but  the  resulting  set  of  fruit  was  small. 

Weather  conditions. — The  weather  conditions  in  1920  were  such  as 
to  bring  about  a  long  blossoming  season.  During  the  last  half  of 
February,  a  succession  of  cold  cloudy  days  occurred  which  held  back 
the  later  varieties.  The  first  variety  to  bloom  was  Formosa,  which 
began  to  open  February  11.  The  last  variety  was  President,  upon 
which  the  last  pollinations  were  made  on  March  19.  At  Newcastle, 
in  1921,  the  season  was  propitious  for  a  heavy  set  of  fruit.  The  polli- 
nation season  commenced  with  the  Formosa  variety  on  February  26 
and  ended  March  9  with  the  California  Blue.  During  the  entire 
season  the  weather  was,  on  the  average,  fairly  warm.  There  were  a 
few  foggy  days,  however,  with  a  light  shower  on  March  5.  In  1922 
the  season  was  the  latest  that  had  been  known  for  many  years.  The 
Wickson,  which  ordinarily  blossoms  the  last  week  in  February,  did 
not  open  until  March  10.  The  California  Blue,  which  blossomed  in 
1921  on  March  9,  did  not  open  until  April  5.  Throughout  the  season 
the  weather  was  cold  and  rainy.  Three  inches  of  snow  fell  during  the 
night  of  March  10.  The  month  of  April  was  marked  by  a  long  period 
of  severe,  drying  northwest  winds.  In  general,  the  weather  was  too 
cold  and  wet  for  the  bees  to  work  upon  the  flowers  and  these  insects 
remained  in  the  hive  for  the  greater  part  of  the  time.  It  is  also  prob- 
able that  the  unseasonably  low  temperatures  affected  the  pollen,  which 
was  scanty  in  many  varieties  and  showed,  on  the  average,  poor 
germinating  power. 

PRESENTATION    OF    DATA 

Germination  of  pollen. — The  pollen  used  was  gathered  from  well 
developed  buds  a  day  or  two  before  it  was  evident  that  they  would 
open.  It  was  carefully  dried  and  kept  in  loosely  stoppered  glass  vials 
until  used.  In  all  cases  the  pollen  was  used  as  soon  after  drying  as 
possible.  Likewise  the  blossoms  were  pollinated  within  a  day  or  two 
after  emasculation.  Blossoms  which  had  been  emasculated  four  or 
five  days  were  discarded  in  favor  of  those  more  recently  prepared  for 
pollination.    All  pollen  used  was  tested  for  viability.    The  percentage 


BULLETIN  352  ]       FURTHER  EXPERIMENTS  IN  PLUM  POLLINATION  249 

of  germination  seemed  to  be  closely  related  to  the  general  character 
of  the  weather  at  and  slightly  before  the  blossoming  season.  During 
1920  and  1921,  when  the  weather  was  generally  warm  and  clear  while 
the  trees  were  in  blossom,  the  pollen  from  the  different  varieties 
showed  a  high  percentage  of  germination.  The  character  of  the  pollen 
used  these  two  seasons  was  reflected  in  the  excellent  results  secured 
from  many  combinations.  On  the  other  hand,  the  1922  season  was 
very  cold  and  rainy,  and  the  pollen  was  scanty  and  showed  poor 
germinating  power.  The  resulting  sets  in  many  cases  were  not  so 
high  as  similar  crosses  made  the  two  previous  seasons. 

Normal  set  of  plums. — "Normal  set"  is  the  term  used  to  designate 
the  percentage  of  flowers  which  set  and  mature  fruit  when  pollinated 
by  chance,  i.e.,  by  insects  or  wind.  It  is  that  set  produced  naturally 
under  open  orchard  conditions  without  artificial  assistance.  The 
normal  set  of  a  variety  is  used  as  the  basis  for  judging  the  results  of 
hand  pollination.  It  also  gives  an  idea  as  to  the  relative  numbers  of 
pollen-carrying  insects  present,  as  it  has  been  repeatedly  observed 
that  the  percentage  of  set  is  higher  when  these  insects  are  plentiful 
than  when  they  are  scarce.  In  cold  or  rainy  seasons,  however,  as  was 
the  case  in  1922,  there  may  be  an  abundance  of  bees  present  without 
a  corresponding  increase  in  the  crop  because  the  bees  are  not  active 
under  these  conditions.  The  normal  sets  for  the  varieties  tested 
during  1920,  1921,  and  1922  are  given  in  table  1. 

The  percentage  of  fruit  set  as  indicated  under  the  columns  "Per- 
centage matured"  does  not  tell  the  whole  story.  A  small  percentage 
of  set  on  varieties  that  have  a  profuse  bloom  often  produces  a  heavier 
crop  than  a  large  percentage  of  set  on  varieties  having  a  small 
number  of  blossoms.  For  example,  in  1920  the  Apex  (often  called  a 
plumcot)  with  a  somewhat  scattering  bloom  and  a  7.5  per  cent  set 
nevertheless  matured  a  medium  crop  of  fruit.  Beauty,  with  15.5  per 
cent  set  in  1920  at  Vacaville,  required  heavy  thinning.  In  1921,  with 
1.6  per  cent  set,  the  crop  was  fair,  but  in  1922,  with  2.4  per  cent  set, 
it  was  very  light,  due  to  the  smaller  number  of  blossoms  produced  on 
each  tree.  The  Burbank  crop  in  1922,  as  a  result  of  an  average  set 
of  6.3  per  cent,  required  heavy  thinning.  This  variety  usually 
blossoms  profusely  and  a  satisfactory  crop  may  be  obtained  from  a 
comparatively  light  set.  Duarte  produced  a  fair  crop  with  4.0  per  cent 
set  in  1921,  but  only  a  light  one  in  1922  when  the  set  dropped  to 
1.2  per  cent.  El  Dorado  produced  a  light  crop  in  1922  as  a  result  of 
a  2.3  per  cent  set. 


250 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Formosa,  as  a  rule,  blossomed  rather  lightly  and  set  erratically. 
In  three  years'  trial  it  was  noticed  that  a  set  of  more  than  5  per  cent 
gave  a  satisfactory  commercial  crop,  while  if  the  set  were  below  that 
figure  the  trees  did  not  carry  as  many  plums  as  they  were  capable  of 

TABLE  1 

Normal  Set  of  Fruit  on  Plum  Varieties  at  Vacaville  in  1920  and  at 
Newcastle  in  1921  and  1922 


1920 

1921 

1922 

Variety 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

Apex 

2311 

2085 

175 
323 

7.5 
15.5 

Beauty 

1652 

27 

1.6 

1552 
1882 
2617 
567 
1982 
2056 
2030 
1926 

38 

119 

32 

13 

178 
83 
53 
59 

2.4 

Burbank 

6.3 

Duarte  ... 

2227 

89 

4.0 

1.2 

El  Dorado 

2.3 

Formosa 

1997 
1742 
2210 

8 
128 
191 

0.4 
7.3 
8.6 

1697 
2486 
1114 

13 
59 
13 

0.8 
5.4 
1.2 

9.0 

Formosa 

4.0 

Gaviota 

2.6 

Kelsey 

3.1 

Methley 

2268 
2063 
915 
2599 
2101 
2198 

709 
110 

10 
135 
215 

98 

31.0 
5.3 
1.1 
5.2 

10.2 
4.1 

Prize 

Santa  Rosa 

1269 
2162 

69 
52 

5.4 
2.4 

2047 

64 

3.1 

Santa  Rosa 

Upright 

Wickson 

2283 

57 

2.5 

California  Blue 

878 
2002 

287 
31 

32.7 
1.6 

1311 

1558 

847 

2310 

343 

10 

186 

136 

26.0 

Diamond  

0.6 

1468 

986 

1440 

427 

72 
68 

29.1 
7.3 

4.8 

22.0 

Grand  Duke 

1860 

95 

5.1 

5.9 

Pond  (Gros) 

1447 

2258 

173 

48 

11.9 

President    

1827 

59 

3.2 

1884 
1112 

63 
6 

3.3 
0.5 

2.1 

Standard 

Tragedy 

1580 
2077 

127 
18 

8.1 
0.9 

1751 

3 

0.2 

2110 

t 
7 

0.3 

maturing.  The  crop  resulting  from  a  9.0  per  cent  set  in  1922  was  the 
largest  which  had  been  obtained  on  this  variety  for  many  years. 
Gaviota,  with  8.6  per  cent  set,  produced  an  excellent  crop  at  Vacaville 
in  1920,  but  in  the  two  following  seasons  at  Newcastle,  sets  of  1.2  per 
cent  and  2.1  per  cent,  respectively,  produced  only  light  crops.  Kelsey 
bore  a  fairly  heavy  crop  with  3.1  per  cent.    Methley  was  tested  only 


BULLETIN  352  ]       FURTHER  EXPERIMENTS  IN  PLUM  POLLINATION 


251 


one  year  and  set  31.0  per  cent.  Observations  have  further  shown  this 
variety  to  be  a  heavy  and  regular  producer.  Prize  produced  a  satis- 
factory crop  with  a  set  of  5.3  per  cent.  Santa  Rosa  in  three  years' 
trials  was  found  to  require  a  set  of  at  least  5  per  cent  to  produce  a 
profitable  crop.  This  variety  is  inclined  to  drop  heavily  before  ripen- 
ing, so  that  an  apparently  heavy  set  early  in  the  season  does  not  give 
as  high  a  yield  as  expected.    Upright  produced  an  excellent  crop  with 


Fig.  1. — a.  The  California  Blue  is  self- fertile  and  sets  fruit  abundantly  with 
its  own  pollen,  b.  A  typical  cluster  of  Formosa  plums  resulting  from  crossing  with 
Wickson. 


a  10.2  per  cent  set.  Wickson  bore  a  satisfactory  crop  with  a  4.1  per 
cent  set  in  1920,  but  a  2.5  per  cent  in  1922  did  not  produce  as  heavy 
a  crop  as  the  trees  were  capable  of  maturing. 

In  the  lower  part  of  the  table  where  the  European  kinds  are  listed, 
California  Blue  is  shown  to  be  an  excellent  bearer.  Observation  of 
this  variety  in  other  orchards  further  substantiated  these  data.  In 
1921  and  in  1922  the  bloom  of  Diamond  was  fairly  profuse,  but  the 
percentages  of  set  were  low.  The  resulting  crops  hardly  paid  for  har- 
vesting. Giant,  in  the  two  years '  trials,  gave  a  high  percentage  of  set 
each  time.  This  variety,  like  the  California  Blue,  has  a  reputation  for 
heavy  bearing.    Imperial  in  one  year's  test  gave  a  fair  crop  with  a 


252  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

set  of  4.8  per  cent,  and  Pond  (Gros  prune)  produced  a  heavy  crop 
with  a  set  of  11.9  per  cent.  For  three  years  the  set  obtained  on 
President  averaged  a  trifle  less  than  3  per  cent.  The  crops  from  tnis 
percentage  of  set  were  fair,  but  not  so  heavy  as  the  trees  could  have 
matured.  Quackenboss  set  only  0.5  per  cent,  which  gave  a  very  light 
crop.  Standard  in  one  year's  trial  gave  a  fair  yield  with  a  set  of 
8.1  per  cent.  Although  Tragedy  blossomed  heavily,  the  average  set 
was  less  than  one  per  cent  in  each  of  the  three  years  the  experiments 
were  conducted.  This  variety  has  long  been  considered  a  shy  bearer 
in  many  sections  of  California. 

Results  of  self-pollination. — Profitable  production  of  plums  is 
closely  correlated  with  the  self -sterility  or  self -fertility  of  the  variety 
in  question.  Results  during  the  past  three  years  as  well  as  those 
reported  of  previous  years  show  decisively  that  most  Japanese  and 
many  European  plums  are  self -sterile  and  need  cross-pollination.  The 
need  for  cross-pollination  is  indicated  in  table  2. 

A  study  of  table  2  shows  that  three  Japanese  plums,  Beauty, 
Methley,  and  Santa  Rosa,  of  the  eleven  varieties  tested  were 
capable  of  setting  a  limited  amount  of  fruit  in  certain  years  with  their 
own  pollen.  To  this  list  may  be  added  Climax,  which  was  previously 
reported  as  being  self -fertile.  It  must  not  be  inferred,  however,  that 
it  is  eminently  satisfactory  to  plant  even  these  varieties  in  large 
blocks  of  one  kind.  More  regular  crops  may  be  expected  when  they 
are  interplanted  with  other  varieties  of  the  same  species. 

Some  of  the  European  plums  are  distinctly  self -sterile,  some  are 
self -fertile,  and  the  remainder,  although  able  to  set  and  mature  a 
limited  amount  of  fruit  when  self -pollinated,  are  greatly  benefited  by 
cross-pollination.  The  Imperial,  President,  Quackenboss,  Standard, 
Tragedy,  and  Washington  gave  distinct  evidence  of  being  self -sterile. 
Although  usually  considered  self -sterile,  Pond  (Hungarian  or  Gros 
prune)  was  able  to  set  a  small  crop  with  its  own  pollen  in  1922. 
Grand  Duke,  in  two  years  out  of  three,  showed  that  it  was  able  to 
set  a  small  percentage  of  blossoms  with  its  own  pollen.  On  the  other 
hand,  California  Blue  (fig.  1,  a),  Giant,  and  Yellow  Egg  produced 
abundant  crops  with  their  own  pollen.  The  experimental  data 
obtained  with  these  three  varieties  were  substantiated  by  the  obser- 
vation of  many  growers,  who  have  noted  that  they  are  consistently 
heavy  bearers.  The  status  of  the  Diamond,  which  is  one  of  the 
important  blue  plums,  was  not  definitely  settled.  In  1920  the  set 
from  self-pollination  was  15.8  per  cent ;  while  in  1921  and  in  1922  at 
Newcastle  the  evidence  pointed  to  distinct  self -sterility.     In  view  of 


BULLETIN  352  ]       FURTHER  EXPERIMENTS  IN  PLUM  POLLINATION 


253 


the  fact  that  this  variety  has  a  reputation  for  shy  bearing,  the  safest 
course,  probably,  is  to  plant  it  with  other  mid-season  kinds,  as  dis- 
cussed later. 

TABLE  2 

Self-Pollination  of  Plum  Varieties  at  Vacaville  in  1920  and  at 
Newcastle  in  1921  and  1922 


1920 

1921 

1922 

Variety 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

Apex 

326 
234 

0 
0 

0 
0 

Beauty 

371 
471 
196 
427 
345 

17 
0 
0 
1 
0 

4.6 
0 
0 

0.2 
0 

258 
324 

8 

0 

3  1 

Duarte.. 

0 

El  Dorado 

Formosa 

861 
432 

0 
0 

0 
0 

308 
280 
173 

0 
0 
0 

0 

Gaviota 

0 

Kelsey 

0 

Methley 

370 
300 

478 
317 

33 
0 
4 
0 

8.9 
0 

0.8 
0 

Prize 

Santa  Rosa 

458 

30 

6.5 

261 

3 

1.2 

Upright 

California  Blue 

328 
612 

53 

1 

16.2 
0.2 

274 
310 
139 

427 

37 
0 

15 
0 

13.5 

Diamond 

310 
291 
470 
231 
259 
355 

49 
9 
7 
0 
0 
0 

15.8 

3.1 

1.5 

0 

0 

0 

0 

Giant 

10.8 

Grand  Duke 

315 

4 

1.3 

0 

Imperial .« 

Pond  (Gros) 

50 
185 
109 

2 
0 
0 

4.0 

President 

524 

556 

0 

0 
0.2 

0 

Quackenboss 

0 

330 
359 
411 
219 

0 

1 

0 
25 

0 

0.3 

0 

11.4 

Tragedy 

202 

0 

0 

216 

0 

0 

Washington 

Yellow  Egg 



Results  of  cross-pollination. — On  the  whole,  the  results  of  the 
crosses  were  decisive.  This  was  particularly  true  in  the  1921  season, 
while  at  Vacaville,  the  preceding  season,  there  was  doubt  as  to  the 
effectiveness  of  a  few  combinations.  From  the  number  of  crosses  with 
each  variety  during  the  three  seasons,  it  is  a  comparatively  simple 
matter  to  choose  combinations  that  will  cross-pollinate  effectively. 
The  results  follow  in  table  3. 

The  general  results  obtained  are  so  evident  from  an  examination 
of  the  table  that  it  seems  unnecessary  to  discuss  each  cross  in  detail. 
It  is  important,  however,   to  point  out  certain  features  regarding 


254 


UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 


several  varieties  about  which  there  has  been  much  complaint,  and  also 
to  discuss  some  cases  in  which  the  results  of  the  experiments  were  not 
satisfactory.  Complaint  regarding  Japanese  varieties  has  centered 
chiefly  on  the  Formosa  and  the  Gaviota.  The  three  years'  work 
showed  that  the  Formosa  may  be  effectively  pollinated  by  Beauty, 
Burbank,  Santa  Rosa,  and  Wickson  (fig.  1,  6).    The  Gaviota  was  most 


Fig.  2. — a.  A  cluster  of  Gaviota  plums  resulting  from  crossing  with  the  Beauty. 
The  Gaviota,  however,  could  not  be  pollinated  with  the  Formosa,  b.  Eight  Tragedy 
plums  set  from  thirty-two  blossoms  pollinated  with  Grand  Duke  pollen.  The 
blossoming  seasons  of  these  two  varieties  usually  overlap  to  a  sufficient  extent  to 
make  this  combination  successful. 


effectively  crossed  by  Beauty  (fig.  2,  a),  Burbank,  Duarte,  and  Wick- 
son. Among  other  important  early  plums,  both  Beauty  and  Santa 
Rosa  were  successfully  pollinated  by  several  Japanese  varieties  and 
also  by  Tragedy.  The  success  or  failure  of  the  crosses  for  the  remain- 
ing varieties  is  easily  determined  from  the  column  in  table  3  entitled 
"Percentage  matured.' ' 


BULLETIN  352  ]       FURTHER  EXPERIMENTS  IN  PLUM  POLLINATION 


255 


TABLE  3 

Cross-Pollination  of  Plum  Varieties  at  Vacaville  in  1920  and  at 
Newcastle  in  1921  and  1922 


1920 

1921 

1922 

Cross 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

Apex 

X  Beauty 

347 
346 
269 
351 
389 
392 

180 

284 

2 

3 
0 
0 
0 

1 

20 
64 

0.6 

0.8 

0 

0 

0 

0.3 

11.0 
22.5 

X  Burbank 

X  Formosa 

X  Gaviota 

X  Santa  Rosa 

X  Tragedy 

Beauty 

X  Apex 

X  Burbank 

335 

67 

20.0 

X  Duarte 

203 
266 
258 
221 
373 
290 
300 

6 
7 
0 
9 
6 
6 
11 

3.0 

X  El  Dorado 

2.6 

X  Formosa 

321 
300 
300 
379 
308 

332 

39 
22 

7 
63 
23 

7 

12.1 
7.3 
2.3 

16.6 

7.5 

2.1 

297 
349 
371 
352 
301 

51 
48 
13 
43 
65 

17.2 
13.8 
3.5 
12.2 
21.6 

0 

X  Gaviota 

4.1 

X  Santa  Rosa 

1.6 

X  Tragedy 

2.1 

X  Wickson 

3.6 

Burbank 

X  Beauty 

X  Duarte 

445 
343 
226 
206 

15 
5 

8 
11 

3.4 

X  Formosa 

1.4 

X  Gaviota 

3.5 

X  Kelsey 

5.3 

X  Methley 

247 

1 

.4 

X  Santa  Rosa 

430 

162 

37.7 

X  Tragedy 

327 

12 

3.7 

Duarte 

X  Beauty 

189 
180 
148 

24 
5 
9 

12.7 

.8 
6.1 

323 
313 
281 
228 
297 
289 
298 
332 
232 

17 
0 
0 
1 
0 
0 
39 
20 
0 

5.3 

X  Burbank 

0 

X  El  Dorado 

0 

X  Formosa 

0.3 

X  Gaviota 

19 

33 

16.6 

0 

X  Kelsey  

0 

X  Santa  Rosa 

13.1 

X  Wickson 

6.0 

X  Tragedy 

0 

256 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE  3—  (Continued) 


1920 

1921 

1922 

Cross 

No.  of 

blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 
matured 

Per- 
centage 
matured 

El  Dorado 

X  Burbank 

113 
117 
127 
106 

4 

0 

12 

10 

3.5 

X  Formosa 

0 

X  Santa  Rosa 

9.4 

X  Wickson 

9.5 

Formosa 

X  Apex 

319 
291 

19 
20 

6.0 
6.9 

X  Beauty 

474 
183 
321 

25 
33 
17 

5.3' 
18.0 
5.3 

97 
336 

349 
....284 

0 
11 
4 
0 
0 
2 
17 

0 

X  Burbank 

3.3 

X  Duarte 

1.1 

X  El  Dorado 

0 

X  Gaviota 

321 

441 

1 
48 

0.3 
10.9 

312 
524 
400 

0 
49 
14 

0 
9.3 
3.5 

318 
332 
314 

0 

X  Santa  Rosa 

0.6 

X  Tragedy 

5.4 

X  Upright 

186 
325 

315 
320 

6 
10 

3.2 
3.1 

X  Wickson 

Gaviota 

X  Apex 

X  Beauty 

X  Burbank 

369 

15 

4.1 

336 

22 

6.6 

212 
286 
249 
253 
328 
293 
256 

26 
0 
6 
1 
0 
12 
21 

12.3 

0 

X  Duarte 

2.4 

X  El  Dorado 

0.4 

X  Formosa 

218 

187 

4 

4 

1.8 
2.1 

436 

0 

0 

0 

X  Santa  Rosa 

4.1 

X  Tragedy 

349 

9 

2.6 

8.2 

X  Upright 

272 
317 

16 

18 

5.9 
5.7 

X  Wickson 

303 

25 

8.3 

235 

202 
200 
197 
71 
206 

114 

10 

11 

5 
2 
0 
5 
3 
2 

4.3 

Kelsey 

X  Beauty 

5.5 

X  Duarte 

2.5 

X  El  Dorado  . 

1.0 

X  Formosa 

0 

X  Gaviota 

2.4 

X  Santa  Rosa 

1.4 

x  Trasredv 

1.7 

BULLETIN  352  ]       FURTHER  EXPERIMENTS  IN  PLUM  POLLINATION 


257 


TABLE  3—  (Continued) 


1920 

1921 

1922 

Cross 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 
matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

Methley 

X  Apex 

285 
149 
34S 
395 
379 

311 
352 

43 
19 
84 
93 
128 

0 
0 

15.0 

12.7 
24.0 
23.6 
33.8 

0 
0 

X  Beautv 

X  Burbank 

X  Tragedy 

X  Wickson 

Santa  Rosa 

X  Apex 

X  Beauty 

192 

16 

8.4 

192 
333 
332 
159 
302 
264 
232 

0 
2 
0 
0 
4 
0 
0 

0 

X  Burbank 

0  6 

X  Duarte 

0 

X  El  Dorado 

0 

X  Formosa 

943 

331 

7 
11 

0.8 
3.3 

375 

24 

6.4 

1.3 

X  Gaviota 

0 

X  Tragedy 

254 

6 

2.4 

0 

X  Upright 

325 

271 

292 
290 
307 
295 

291 

2 
3 

7 
24 

2 
2 

2 

0.6 
1.1 

2.4 
8.3 
0.7 
0.7 

0.7 

X  Wickson 

271 

30 

11.1 

308 

4 

1.3 

Upright 

X  Formosa 

X  Gaviota 

X  Santa  Rosa  . . 

X  Wickson 

Wickson 

X  Beauty 

103 
215 
336 
265 
353 

4 

1 

4 

15 

25 

3.9 

X  Duarte 

0.5 

X  Formosa 

317 
308 
307 
350 
293 

14 
17 

6 
42 

3 

4.4 
5.5 
1.9 
12.0 
1.0 

1.2 

X  Gaviota 

5.7 

X  Santa  Rosa 

7.1 

X  Tragedy 

X  Upright 

California  Blue 
X  Diamond  ... 

258 
288 

64 
13 

24.8 
4.5 

X  Gr.  Duke 

X  Pond  (Gros) 

142 

4 

2.8 

X  President 

176 

86 

21 
12 

11.9 
13.9 

X  Quackenboss 

258 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE  Z—(Co?itinued) 


1920 

1921 

1922 

Cross 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

Diamond 

X  California  Blue 

239 
356 

81 
118 

33.9 
33.1 

X  Grand  Duke 

298 
305 
223 

49 
15 

18 

16.5 

4.9 

8.1 

339 

30 

8.8 

X  Imperial 

X  President 

383 

466 

60 

188 

15.7 
40.4 

328 
405 
315 

158 

8 

0 

84 

55 

2.4 

X  Quackenboss 

0 

X  Tragedy 

26.7 

Giant 

X  California  Blue 

34.8 

X  Gr.  Duke 

133 
226 
310 

8 
2 

27 

6.1 
0.9 

8.7 

X  Pond  (Gros) 

96 
257 

5 
51 

5.2 

X  President... 

19.8 

Grand  Duke 

X  California  Blue  . 

352 
325 

108 

87 

30.7 
26.8 

X  Diamond 

X  Giant 

504 
263 

131 
3 

26.0 
1.1 

274 

14 

5.1 

X  President 

411 
303 

82 
67 

19.9 
22.1 

363 

268 

34 
3 

9.4 

X  Quackenboss 

1.1 

X  Standard 

608 

145 

23.9 

X  Tragedy 

318 

75 

23.6 

302 

111 
255 

36 

6 

28 

11.9 

Pond  (Gros) 

X  California  Blue 

14.4 

X  Giant 

224 
124 

1 
2 

0.5 
1.6 

11.0 

X  Gr.  Duke 

President 

X  California  Blue 

292 
483 

109 
129 

37.3 
26.7 

X  Diamond 

295 
323 
291 
323 

0 

23 

0 

18 

0 
7.1 

0 
5.6 

333 
121 
217 
101 
433 
461 

1 
4 
3 
4 
0 
15 

0.3 

X  Giant 

3.3 

X  Gr.  Duke 

350 

82 

23.4 

1.4 

X  Pond  (Gros) 

4.0 

X  Quackenboss 

457 

112 

24.5 

0 

X  Tragedy 

3.2 

Quackenboss 

X  California  Blue 

233 
555 
532 
369 

89 
106 
108 

47 

38.2 
19.1 
20.3 
12.7 

86 

94 

173 

1 
3 

27 

1.2 

X  Gr.  Duke 

3.2 

X  President.... 

15.6 

Bulletin  352  ]     further  experiments  in  plum  pollination 


259 


TABLE  3—  (Concluded) 


1920 

1921 

1922 

Cross 

'No.  of 
blos- 
soms 
used 

331 
320 

334 

363 
389 

372 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 

matured 

Per- 
centage 
matured 

No.  of 
blos- 
soms 
used 

No.  of 

fruits 
matured 

Per- 
centage 
matured 

Standard 

X  Diamond 

35 
49 
13 

0 
0 
0 

10.6 

15.3 

3.9 

0 
0 
0 

X  Gr.  Duke 

X  Imperial 

Tragedy 

X  Apex 

X  Beauty 

298 
271 

0 
0 

0 
0 

271 
225 
357 
301 
306 
247 
355 
264 

0 
0 
24 
0 
0 
0 
113 
0 

0 

X  Burbank 

0 

X  Diamond 

6.7 

X  Duarte 

0 

X  El  Dorado 

0 

X  Formosa 

337 
323 

0 
171 

0 
53.0 

0 

X  Gr.  Duke 

517 

12 

2.3 

31.8 

X  Kelsey 

0 

X  Methley 

344 

0 

0 

X  Santa  Rosa 

291 

298 

0 
0 

0 
0 

207 

258 

0 
0 

0 

X  Wickson 

366 

1 

0.3 

0 

It  is  also  important  to  note  some  of  the  questionable  combinations. 
Apex  in  1920  proved  to  be  an  uncertain  pollinator  for  Formosa  and 
Santa  Rosa  and,  in  turn,  was  ineffectively  pollinated  by  them.  El 
Dorado,  Formosa,  Gaviota,  and  Kelsey,  in  general,  were  scanty  pollen 
producers  and  were  therefore  not  reliable  pollinators  for  other 
Japanese  varieties.  On  the  other  hand,  Beauty,  Burbank,  Duarte, 
Methley,  Santa  Rosa,  and  Wickson  produced  a  large  amount  of  viable 
pollen  and  have  proved  to  be  excellent  pollinators.  One  outstanding 
result  was  the  apparent  inter-sterility  of  Formosa  and  Gaviota  which 
during  three  years'  trials  set  very  poorly  when  crossed.  This  was  the 
first  case  of  inter-sterility  found  among  Japanese  plums. 

Tragedy,  for  three  years,  was  found  to  pollinate  effectively  the 
different  Japanese  varieties.  When  the  reciprocal  cross  was  tried, 
failure  resulted  in  every  case :  the  Japanese  varieties  were  unable  to 
cause  the  Tragedy  to  set  fruit.  From  a  practical  standpoint  this  fact 
is  unfortunate,  as  Tragedy  is  self -sterile  and  usually  blossoms  too  soon 
to  be  pollinated  by  the  early  blossoming  European  plums.  The 
inability  of  the  Japanese  varieties  to  pollinate  Tragedy  may  be  due 


260 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


to  the  length  of  the  pistil  of  the  latter.  According  to  Knight,*  lack  of 
fertilization  is  sometimes  due  to  the  fact  that  the  pollen  tube  does 
not  grow  down  the  style  and  fertilize  the  ovule  before  the  latter  is 
beyond  the  receptive  stage.  Evidently  Tragedy  pollen  grows  down 
the  comparatively  short  pistils  of  the  Japanese  sorts  quickly  enough 
to  fertilize  the  ovules  before  they  begin  to  disintegrate,  but  in  the 


Fig.  3. — a.  A  typical  cluster  of  Diamond  plums  resulting  from  crossing  with 
the  Tragedy.  Satisfactory  results  were  also  obtained  by  crossing  the  Diamond 
with  the  Grand  Duke.  b.  Typical  branch  from  the  Beauty  tree  inclosed  with  a 
hive  of  bees  in  a  mosquito-bar  tent  with  a  Santa  Rosa  tree.  The  stems  without 
fruits  indicate  where  plums  which  set  originally  were  removed  in  thinning. 

reciprocal  cross  the  growth  may  be  so  slow  that  fertilization  does  not 
take  place.  Tragedy,  however,  was  very  effectively  pollinated  by  both 
Grand  Duke  (fig.  2,  b)  and  Diamond,  although  in  some  seasons  it  was 
difficult  to  get  this  combination,  due  to  the  differences  in  blossoming 
season. 

In  the  Placer  County  district,  Diamond  has  long  been  known  as  a 
shy  bearer,  but  on  account  of  its  excellent  shipping  qualities  growers 
were  loath  to  graft  it  over.    From  the  data  it  can  readily  be  seen  that 


*  Physiological  Aspects  of  the  Self-Sterility  of  the  Apple. 
Hort.  Sci.,  1917,  p.  101. 


Proc.  Am.  Soc. 


BULLETIN  352]       FURTHER  EXPERIMENTS  IN  PLUM  POLLINATION  261 

this  variety  can  be  made  to  produce  when  cross-pollinated  by  any  one 
of  several  mid-season  European  plums.  The  best  results  were  obtained 
with  Grand  Duke,  President,  California  Blue,  and  Tragedy  (fig.  3,  a). 
As  many  combinations  of  European  varieties  were  tried  each  year  as 
possible.  In  no  case  was  there  any  evidence  of  inter-sterility,  although 
in  one  or  two  instances  light  yields  resulted  from  certain  crosses  in 
one  of  the  three  trials.  For  example,  Quackenboss  in  1921  was  shown 
to  cross  effectively  with  several  other  mid -season  plums,  but  the  follow- 
ing year,  the  same  combinations  being  used,  the  resulting  sets  were 
low.  Observations  over  a  period  of  three  years  showed  that  the 
European  varieties  reported  in  this  bulletin  produced  pollen  abund- 
antly. Each  variety  could  be  used  to  pollinate  the  others  provided 
the  blossoming  of  both  occurred  at  approximately  the  same  time.  The 
results  obtained  with  this  type  of  plums  as  shown  in  the  preceding 
table  were  marked. 

In  addition  to  the  data  obtained  in  the  experiment  proper,  the 
beneficial  results  of  mixed  plantings  were  clearly  evident  to  even  a 
casual  observer.  In  the  Cook  orchard,  both  in  1921  and  1922,  the  set 
of  fruit  on  different  varieties  was  invariably  best  on  the  rows  where 
two  varieties  adjoined.  Many  reports  received  from  growers  sub- 
stantiate these  observations. 

The  use  of  bees  as  pollen  distributors. — At  Newcastle,  in  1921  and 
1922,  mosquito-bar  tents  (frontispiece)  were  erected  over  adjoining 
trees  of  different  varieties  in  a  manner  similar  to  that  used  in  previous 
experiments  with  prunes  in  the  Santa  Clara  Valley.*  In  1921  one 
pair  of  trees  consisted  of  a  Formosa  and  a  Wickson,  the  other  a 
Formosa  and  a  Gaviota.  A  colony  of  bees  was  kept  in  each  tent  for 
six  or  seven  days  during  the  blossoming  season.  At  first  the  bees  tried 
to  escape,  but  later  went  to  work  on  the  blossoms. 

The  trees  under  the  tents,  as  far  as  could  be  observed,  blossomed 
and  produced  leaves  in  a  normal  manner.  The  only  climatic  differences 
within  the  tent  that  could  be  noticed  as  compared  with  conditions  in 
the  open  orchard  were  a  slightly  reduced  light  intensity  and  a  slower 
air  movement.  It  is  doubtful  that  these  differences  were  great  enough 
to  influence  the  setting  of  the  fruit.    The  results  are  given  in  table  4. 

In  the  case  of  the  Formosa  and  Wickson  trees  the  resulting  set 
was  so  heavy  as  to  be  very  noticeable  even  after  thinning.  The 
Formosa  enclosed  with  bees  and  a  Wickson  tree  matured  7.3  per  cent, 
as  compared  with  0.8  per  cent  which  was  the  average  under  open 
orchard  conditions.    The  Wickson  matured  11.2  per  cent,  which  is  a 

*  The  Common  Honey  Bee  as  an  Agent  in  Prime  Pollination.  Calif.  Agr. 
Exp.  Sta.  Bull.  291. 


262 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


very  heavy  yield  for  that  variety.  Through  an  oversight,  no  counts 
were  made  on  adjoining  normal  Wickson  trees,  so  that  no  comparison 
can  be  made  with  the  yield  obtained  hy  normal  pollination.  Observa- 
tion showed,  however,  that  the  Wickson  tree  with  bees  to  carry  pollen 
from  a  Formosa,  set  the  heaviest  crop  of  all  trees  of  that  variety. 
Throughout  the  orchard  the  crop  of  Formosa  was  heavier  on  the  row 
of  trees  adjoining  the  Wickson  than  in  the  interior  of  the  block. 

TABLE  4 

Behavior  of  Trees  Enclosed  under  a  Mosquito-Bar  Tent  with  a 
Hive  of  Bees  in  1921 


Variety 


Formosa: 
Formosa: 
Wickson: 
Formosa: 
Gaviota: 
Gaviota: 


Average  set  in  open  orchard 

Enclosed  with  bees  and  a  Wickson  tree 
Enclosed  with  bees  and  a  Formosa  tree 
Enclosed  with  bees  and  a  Gaviota  tree. 
Enclosed  with  bees  and  a  Formosa  tree 
Average  set  in  open  orchard 


No.  of 

blossoms 

used 

No.  of 

fruits 

matured 

1697 

13 

1646 

120 

1513 

169 

1148 

2 

1119 

14 

1114 

13 

Per- 
centage 
matured 


0.8 
7.3 
11.2 
0.2 
1.3 
1.2 


In  the  experiment  with  Formosa  and  Gaviota  trees  no  increase  in 
crop  was  produced.  The  Formosa  tree  actually  set  a  smaller  per- 
centage of  fruit  than  the  average  for  the  trees  of  this  variety  in  the 
open.  Together  with  the  data  obtained  from  hand  pollination,  the 
results  on  these  trees  showed  that  the  Formosa  and  Gaviota  are 
apparently  inter-sterile  or  are  at  least  unsafe  for  interplanting  for 
pollination  purposes. 

In  1922  a  Beauty  and  a  Santa  Kosa  tree  were  enclosed  in  one  tent, 
and  a  Grand  Duke  and  a  Diamond  in  another.  A  hive  of  bees  was 
kept  in  each  tent  for  six  or  seven  days  during  the  blossoming  season, 
as  was  done  the  preceding  season.    The  results  are  given  in  table  5. 

The  Beaut}^  (fig.  3,  b)  tree  in  this  trial  set  a  very  heavy  crop  which 
had  to  be  thinned  severely.  The  percentage  of  set  was  approximately 
four  times  as  high  as  the  average  percentage  of  set  in  the  orchard 
exposed  to  chance  pollination.  The  Santa  Rosa  tree  in  the  tent  failed 
to  respond  as  expected,  but  the  difference  in  percentage  of  set  as 
compared  with  other  Santa  Rosa  trees  in  the  open  was  in  favor  of  the 
tree  in  the  tent. 

In  the  second  tent,  the  percentages  of  set  for  both  the  enclosed 
Diamond  (fig.  4)  and  the  Grand  Duke  trees  as  compared  with  the 
orchard  average  showed  conclusively  the  value  of  bees  in  distributing 


BULLETIN  352  ]       FURTHER  EXPERIMENTS  IN  PLUM  POLLINATION 


263 


pollen  from  one  variety  to  another.  The  light  crops  produced  under 
open  orchard  conditions  were  undoubtedly  due  to  lack  of  pollen 
distribution,  as  but  few  honey  bees  were  present  in  the  neighborhood 
and  only  rarely  were  they  seen  on  the  trees  adjoining  those  used  in 
the  tent  experiment. 

TABLE  5 

Behavior  of  Trees  Enclosed  under  a  Mosquito-Bar  Tent  with  a 
Hive  of  Bees  in  1922 


Variety- 


Beauty: 
Beauty: 
Santa  Rosa: 
Santa  Rosa: 
Diamond: 
Diamond: 
Grand  Duke: 
Grand  Duke: 


Average  set  in  open  orchard 

Enclosed  with  bees  and  a  Santa  Rosa  tree  . 

Average  set  in  open  orchard 

Enclosed  with  bees  and  a  Beauty  tree 

Average  set  in  open  orchard 

Enclosed  with  bees  and  a  Grand  Duke  tree. 

Average  set  in  open  orchard 

Enclosed  with  bees  and  a  Diamond  tree 


No.  of 

blossoms 

used 

No.  of 

fruits 

matured 

2297 

87 

2532 

432 

2273 

17 

2130 

38 

1759 

17 

1386 

249 

2310 

136 

1533 

213 

Per- 
centage 
matured 


3.8 

17.1 

0.7 

1.8 

0.9 

18.0 

5.9 

13.9 


Blossoming  dates. — The  time  of  blossoming  of  practically  all 
varieties  given  in  this  report  showed  a  great  deal  of  variation  during 
the  three  years  of  observation.  In  1922  the  season  was  practically 
four  weeks  later  than  it  was  in  1921.  The  varieties  opened  in  approxi- 
mately the  same  order  both  years.  The  1920  season  was  different  from 
the  other  two  in  that  the  first  varieties  blossomed  earlier  than  usual 
but  the  last  ones  opened  later  than  was  expected.  In  other  words,  the 
season  was  exceptionally  long,  and  distinct  gaps  occurred  between 
many  varieties  that  ordinarily  overlap  each  other. 

Three  years'  observation  has  indicated  that  certain  varieties  of 
plums  included  in  this  report  could  be  placed  in  groups  blossoming 
closely  enough  together  for  cross-pollination  purposes.  The  first  group 
would  contain  the  early  blossoming  Japanese  varieties,  which  open  in 
approximately  the  following  order:  Formosa,  Santa  Rosa,  Upright, 
Wickson,  Gaviota,  and  Beauty.  The  second  group  would  include  the 
Duarte,  El  Dorado,  Kelsey,  Apex,  Methley,  Prize,  and  Burbank.  In 
all  seasons  there  was  considerable  overlapping  in  the  blossoming 
season  of  these  varieties.  It  is  therefore  also  possible  to  use  the  early 
members  of  the  second  group  with  the  late  members  of  the  first. 
Such  early  sorts  as  Formosa  or  Santa  Rosa,  however,  could  not  be 
consistently  pollinated  by  such  late  bloomers  as  Methley  or  Burbank. 


264  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Undesirable  combinations  previously  mentioned  should  be  avoided, 
even  though  the  varieties  blossom  together.  Before  planting  the 
grower  should  consult  table  3  to  find  out  if  the  desired  combinations 
have  proved  satisfactory  experimentally. 

The  blossoming  season  of  the  Tragedy  closely  coincides  with  several 
of  the  late  Japanese  varieties.  It  also  usually  extends  into  the 
blossoming  periods-  of  both  the  Grand  Duke  and  Diamond.  The 
remainder  of  the  European  varieties  may  be  placed  in  two  fairly 
distinct  groups  with  relation  to  pollination  season.  The  first  group 
would  include  Grank  Duke,  Diamond,  Quackenboss,  Standard.  Im- 
perial, and  President,  and  the  late  group  California  Blue,  Pond  (Gros 
prune),  Giant,  Yellow  Egg,  and  Washington.  In  certain  seasons 
there  might  be  sufficient  change  in  the  order  of  blossoming  to  permit 
Pond  and  Giant  to  be  pollinated  by  President  or  Standard,  but  in 
normal  seasons  those  varieties  blossoming  later  than  President  are  not 
effectively  pollinated  by  members  of  the  early  group. 


SUMMARY 

1.  In  addition  to  varieties  previously  reported  the  following 
Japanese  plums  were  found  to  be  self  -sterile :  Apex,  Duarte,  El 
Dorado,  Formosa,  Gaviota,  Prize,  and  Upright. 

2.  In  three  years'  experiments,  Formosa  and  Gaviota  were  shown 
to  be  inter-sterile. 

3.  Beauty,  Methley,  and  Santa  Rosa  seemed  to  be  self -fertile,  at 
least  to  a  limited  degree. 

4.  Apex,  El  Dorado,  Formosa,  Gaviota,  and  Kelsey  were  generally 
found  to  be  scanty  pollen  producers  and  therefore  of  questionable 
value  as  pollinators  of  other  Japanese  varieties. 

5.  Beauty,  Burbank,  Duarte,  Santa  Rosa,  and  Wickson  seemed  to 
be  the  most  effective  pollinators  for  the  Japanese  varieties. 

6.  Among  the  European  varieties,  Imperial  and  Tragedy  were 
again  shown  to  be  self-sterile.  Likewise  President,  Quackenboss, 
Standard,  and  Washington  proved  unable  to  set  fruit  with  their  own 
pollen. 

7.  Diamond  set  well  with  its  own  pollen  one  year,  but  failed  to 
do  so  the  other  two.  For  practical  purposes  it  must  be  considered 
self -sterile. 

8.  Pond  (Hungarian  or  Gros  prune)  when  pollinated  with  its  own 
pollen  did  not  set  any  fruit  in  1920,  but  set  a  light  crop  in  1922.  It 
is  probably  partially  self -fertile. 


BULLETIN  352  ]       FURTHER  EXPERIMENTS  IN  PLUM  POLLINATION 


265 


Fig.  4. — Characteristic  branches  of  the  Grand  Duke  and  of  the  Diamond 
from  trees  inclosed  in  a  mosquito-bar  tent  in  which  a  hive  of  bees  was  kept 
during  the  blossoming  season. 


266  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

9.  California  Blue,  Giant,  and  Yellow  Egg  were  shown  to  be 
self -fertile.  Grand  Duke,  contrary  to  results  obtained  at  Davis  where 
it  was  found  to  be  self -sterile  under  certain  conditions,  was  shown  to 
be  able  to  set  a  small  percentage  of  fruit  with  its  own  pollen. 

10.  All  European  plums  blossoming  in  midseason  or  later  appeared 
to  be  able  to  cross-pollinate  effectively. 

11.  Tragedy  was  able  to  pollinate  the  Japanese  varieties  but  was 
not  pollinated  by  them. 

12.  The  presence  of  honey  bees  materially  aided  in  setting  heavy 
crops  on  the  following  combinations  of  varieties ;  Formosa  and  Wick- 
son;  Beauty  and  Santa  Rosa;  Diamond  and  Grand  Duke.  Observa- 
tions, furthermore,  showed  that  many  other  combinations  were  also 
benefited  by  these  insects. 

13.  Inter-pollination  of  Formosa  and  Gaviota  by  means  of  bees 
proved  unsuccessful.  The  interplanting  of  these  two  varieties  for 
cross-pollination  purposes  cannot  be  recommended. 

14.  The  results  showed  that  whenever  a  particular  cross  had  been 
made  for  two  or  three  successive  years  the  percentage  of  set  was 
usually  lowest  in  1922,  a  fact  probably  due  to  the  prolonged  period 
of  cold  rainy  weather  during  the  blossoming  season  of  that  year. 


ACKNOWLEDGMENTS 

The  writer  is  indebted  to  Messrs.  Millard  Sharpe  and  H.  B.  Naylor,. 
of  Vacaville,  to  Messrs.  J.  F.  Dudley,  E.  Van  Riper,  and  William 
Lower,  of  Newcastle,  for  the  use  of  their  orchards  in  conducting  the 
experiments.  Special  thanks  are  due  to  Mr.  Dudley  for  cooperation 
in  furnishing  material  used  in  building  the  mosquito-bar  tents.  To 
Messrs.  E.  W.  Everett,  L.  H.  Henderson,  and  E.  Van  Riper,  advanced 
students  in  Pomology,  the  writer  conveys  his  thanks  and  appreciation 
for  assistance  in  counting  and  pollinating  the  blossoms. 


STATION  PUBLICATIONS  AVAILABLE  FOE  FKEE  DISTRIBUTION 


BULLETINS 

No.  No. 

253.  Irrigation   and   Soil  Conditions  in  the  319. 

Sierra  Nevada  Foothills,  California.  321. 

261.  Melaxuma    of    the    Walnut,     "Juglans  324. 

regia." 

262.  Citrus   Diseases   of   Florida   and   Cuba  325. 

Compared  with  those  of  California. 

263.  Size  Grades  for  Ripe  Olives. 

268.  Growing  and  Grafting  Olive  Seedlings.  328. 

270.  A  Comparison  of  Annual  Cropping,  Bi-  330. 

ennial  Cropping,  and  Green  Manures  331. 

on  the  Yield  of  Wheat.  332. 

273.  Preliminary  Report  on  Kearney  Vine-  334. 

yard  Experimental  Drain. 

275.  The  Cultivation  of  Belladonna  in  Cali-  335. 

fornia. 

276.  The    Pomegranate.  336. 

278.  Grain   Sorghums. 

279.  Irrigation  of  Rice  in  California.  337. 

280.  Irrigation  of  Alfalfa  in  the  Sacramento  339. 

Valley. 

283.  The  Olive  Insects  of  California.  340. 

285.  The  Milk  Goat  in  California. 

286.  Commercial    Fertilizers.  341. 

287.  Vineerar  from  Waste  Fruits.  342. 
294.   Bean    Culture   in   California.  343. 

297.  The  Almond  in  California.  344. 

298.  Seedless  Raisin  Grapes. 

299.  The  Use  of  Lumber  on  California  Farms.  345. 
304.  A  study  on  the  Effects  of  Freezes  on 

Citrus   in   California.  346. 

308.   I.  Fumigation  with  Liquid  Hydrocyanic  347. 
Acid.  II.  Physical  and  Chemical  Prop- 
erties of  Liquid  Hydrocyanic  Acid.  348. 

310.   Plum  Pollination.  352. 

312.  Mariout  Barley. 

313.  Pruning  Young   Deciduous  Fruit  Trees.  353. 

316.  The   Kaki   or   Oriental  Persimmon. 

317.  Selections  of   Stocks  in  Citrus   Propa- 

gation. 


Caprifigs  and  Caprification. 

Commercial  Production  of  Grape  Syrup. 

Storage  of  Perishable  Fruit  at  Freezing 
Temperatures. 

Rice  Irrigation  Measurements  and  Ex- 
periments in  Sacramento  Valley, 
1914-1919. 

Prune  Growing  in  California. 

Dehydration  of  Fruits. 

Phylloxera-Resistant  Stocks. 

Walnut  Culture  in  California. 

Preliminary  Volume  Tables  for  Second- 
Growth  Redwoods. 

Cocoanut  Meal  as  a  Feed  for  Dairy 
Cows  and  Other  Livestock. 

The  Preparation  of  Nicotine  Dust  as 
an  Insecticide. 

Some  Factors  of  Dehydrater  Efficiency. 

The  Relative  Cost  of  Making  Logs  from 
Small    and    Large    Timber. 

Control  of  the  Pocket  Gopher  in  Cali- 
fornia. 

Studies  on  Irrigation  of  Citrus  Groves. 

Hog  Feeding  Experiments. 

Cheese  Pests  and  Their  Control. 

Cold  Storage  as  an  Aid  to  the  Market* 
ing  of  Plums. 

Fertilizer  Experiments  with  Citrus 
Trees. 

Almond    Pollination. 

The  Control  of  Red  Spiders  in  Decidu- 
ous Orchards. 

Pruning  Young  Olive  Trees. 

Further  Experiments  in  Plum  Pollina- 
tion. 

Bovine  Infectious  Abortion. 


CIRCULARS 

No.  No. 
70.  Observations    on    the    Status    of    Corn  172. 
Growing  in  California.  173. 
82.  The  Common  Ground  Squirrel  of  Cali- 
fornia. 174. 
87.  Alfalfa.  175. 

110.  Green   Manuring  in   California. 

111.  The  Use  of  Lime  and  Gypsum  on  Cali-  178. 

fornia  Soils.  179. 
113.   Correspondence  Courses  in  Agriculture. 

126.  Spraying  for  the  Grape  Leaf  Hopper.  181. 

127.  House  Fumigation. 

136.  Melilotus   indica    as    a    Green-Manure  182. 

Crop  for  California. 

144.   Oidium  or  Powdery  Mildew  of  the  Vine.  183. 

148.   "Lungworms."  184. 

151.  Feeding  and  Management  of  Hogs.  188. 

152.  Some  Observations  on  the  Bulk  Hand-  189. 

ling  of  Grain  in   California.  190. 

155.   Bovine  Tuberculosis.  193. 

157.   Control  of  the  Pear  Scab.  198. 

159.  Agriculture  in   the  Imperial   Valley.  199. 

160.  Lettuce  Growing  in  California.  201. 

161.  Potatoes  in   California.  202. 

164.  Small  Fruit  Culture  in   California. 

165.  Fundamentals    of    Sugar   Beet   Culture  203. 

under  California  Conditions.  205. 

166.  The  County  Farm  Bureau.  206. 

167.  Feeding  Stuffs  of  Minor  Importance.  208. 

169.  The   1918   Grain  Crop. 

170.  Fertilizing  California  Soils  for  the  1918  209. 

Crop.  210. 


Wheat  Culture. 

The    Construction    of    the    Wood-Hoop 

Silo. 
Farm   Drainage  Methods. 
Progress  Report  on  the  Marketing  and 

Distribution  of  Milk. 
The  Packing  of  Apples  in  California. 
Factors    of    Importance    in    Producing 

Milk  of  Low  Bacterial  Count. 
Control     of     the     California      Ground 

Squirrel. 
Extending  the  Area  of  Irrigated  Wheat 

in   California  for  1918. 
Infectious  Abortion  in  Cows. 
A  Flock  of  Sheep  on  the  Farm. 
Lambing  Sheds. 
Winter  Forage  Crops. 
Agriculture  Clubs   in  California. 
A  Study  of  Farm  Labor  in  California. 
Syrup  from   Sweet  Sorghum. 
Onion  Growing  in  California. 
Helpful  Hints  to  Hog  Raisers. 
County    Organizations   for   Rural    Fire 

Control. 
Peat  as  a  Manure  Substitute. 
Blackleg. 
Jack  Cheese. 
Summary  of  the  Annual  Reports  of  the 

Farm   Advisors  of  California. 
The  Function  of  the  Farm  Bureau. 
Suggestions  to  the  Settler  in  California. 


CIRCULARS — Continued 


No. 
212. 
214. 

215. 
217. 

218. 

219. 
224. 


225. 
228. 
230. 

232. 

233. 
234. 

235. 

236. 

237. 


No. 

Salvaging  Rain-Damaged  Prunes.  238. 

Seed  Treatment  for  the  Prevention  of  239. 

Cereal   Smuts. 
Feeding  Dairy  Cows  in  California.  240. 

Methods   for   Marketing   Vegetables   in 

California.  241. 

Advanced    Registry    Testing    of    Dairy 

Cows.  242. 

The  Present  Status  of  Alkali.  244. 

Control    of    the    Brown    Apricot    Scale  245. 

and  the  Italian  Pear  Scale  on  Decid-  246. 

uous  Fruit  Trees. 
Propagation  of  Vines.  247. 

Vineyard   Irrigation  in  Arid  Climates.  248. 

Testing  Milk,    Cream,    and    Skim   Milk 

for  Butterfat.  249. 

Harvesting    and    Handling    California  250. 

Cherries  for  Eastern  Shipment. 
Artificial  Incubation.  251. 

Winter  Injury  to  Young  Walnut  Trees 

during  1921-22. 
Soil  Analysis  and  Soil  and  Plant  Inter-  252. 

relations.  253. 

The  Common  Hawks  and  Owls  of  Cali-  254. 

fornia   from    the     Standpoint  of  the 

Rancher.  255. 

Directions  for  the  Tanning  and  Dress- 
ing of  Furs. 


The  Apricot  in  California. 

Harvesting  and  Handling  Apricots  and 
Plums  for  Eastern  Shipment. 

Harvesting  and  Handling  Pears  for 
Eastern   Shipment. 

Harvesting  and  Handling  Peaches  for 
Eastern   Shipment. 

Poultry  Feeding. 

Central  Wire  Bracing  for  Fruit  Trees. 

Vine  Pruning  Systems. 

Desirable  Qualities  of  California  Bar- 
ley for  Export. 

Colonization  and  Rural  Development. 

Some  Common  Errors  in  Vine  Pruning 
and  Their  Remedies. 

Replacing  Missing  Vines. 

Measurement  of  Irrigation  Water  on 
the   Farm. 

Recommendations  Concerning  the  Com- 
mon Diseases  and  Parasites  of 
Poultry    in    California. 

Supports  for  Vines. 

Vineyard   Plans. 

The  Use  of  Artificial  Light  to  Increase 
Winter  Egg  Production. 

Leguminous  Plants  as  Organic  Fertil- 
izer in  California  Agriculture. 


